Delayed coking is a thermal cracking process used in petroleum refineries to upgrade and convert petroleum residuum, which are typically the bottoms from the atmospheric and vacuum distillation of crude oil, into liquid and gas product streams leaving behind petroleum coke as a solid concentrated carbon material. A fired heater or furnace, e.g., of the horizontal tube type, is used in the process to reach thermal cracking temperatures of 485° C. to 505° C./905° F. to 941° F. With a short residence time in the furnace tubes, coking of the feed material is thereby “delayed” until it is discharged into large coking drums downstream of the heater.
In the practice of the delayed coking process, a hydrocarbon oil is heated to a coking temperature in a furnace or other heating device and the heated oil is introduced into a coking drum to produce a vapor phase product, which also forms liquid hydrocarbons, and coke. The drum can be decoked by hydraulic means or by mechanical means. In most configurations of the delayed coking process, the fresh hydrocarbonaceous feed to the coking unit is first introduced into a coker product fractionating column, or fractionator, usually for heat exchange purposes, where it combines with the heavy coker oil products that are recycled as bottoms to the coking unit heater.
It is known that decreasing the recycle ratio of the fractionator bottoms that are recycled to the delayed coker preheater results in an increase in the hydrocarbon liquid yield and a decrease in the coke yield of the delayed coker and, conversely, that as the recycle ratio is increased, the coke yield also increases. Thus, the effect of the recycle ratio to coke yield is such that as recycle decreases, the cut point of the recycle increases. Other operating conditions that effect the delayed coking are drum temperature and pressure. As the temperature is increased, the coke yield decreases and a harder type of coke is produced. An increase in drum pressure produces an increase in the yield of both coke and gases. A delayed coking process is disclosed in U.S. Pat. No. 4,492,625 in which the hydrocarbon feedstock having a boiling point of 925° F./450° C. is split before the preheating step with one portion being sent to the delayed coking unit preheater and a second portion being introduced directly into the coker unit product fractionator. At least a portion of the bottom residue, or bottoms, from this fractionator is recycled to the preheater where it is combined with the fresh hydrocarbon feedstock, and the combined feedstock is heated to a predetermined temperature and passed to the delayed coking unit.
The boiling point of the feedstream employed in the process described in the '625 patent indicates that the hydrocarbon feedstream had been previously upgraded, e.g., by fractional distillation, before its processing in the delayed coking unit and its introduction into the fractionator above the coker unit product feed to the fractionator. There is no significant effect on the capital or operating costs associated with the operation of the product fractionator in this mode. Rather, it is equivalent to the conventional steps of atmospheric distillation followed by vacuum distillation of whole crude oil, followed by coking of the residuum or bottoms.
A process is described in U.S. Pat. No. 4,066,532 for delayed coking in which the fresh feedstock is introduced to a preheating furnace as a mixture with the bottoms and a portion of the heavy gas oil side stream from the coker unit product fractionator, or fractionating column. It is stated that the recycling of the heavy gas oil will result in an increase in the aromaticity of this side stream, a portion of which can advantageously be used for carbon black production. The fresh feedstock is described as including coal tar and decanted cracking oil having prescribed sulfur, ash and asphaltene contents. The temperature of the mixed feedstock is raised to 450° C. to 510° C./842° F. to 950° F. in the preheating furnace.
A catalytically enhanced delayed coking process is described in U.S. Pat. No. 4,394,250 in which from about 0.1% to 3% of catalyst and hydrogen are added to the feedstock before it is introduced into the furnace with a portion of the fractionator bottoms. The feedstock is selected from heavy low-grade oil such as heavy virgin crude, reduced crude, topped crude, and residuums from refining processes.
A problem exists with respect to the utilization of coking units in refinery processes because of the need to use a feedstream that is the product of atmospheric and/or vacuum distillation, which will require either the construction of new distillation facilities for this purpose or an increase of the burden on existing facilities, both of which alternatives will result in an increase in capital and/or operating costs.
Computer models can be used advantageously in evaluating whether process modifications are technically feasible and economically justifiable. The use of computer modeling is described by J. F. Schabron and J. G. Speight in an article entitled “An Evaluation of the Delayed-Coking Product Yield of Heavy Feedstocks Using Asphaltene Content and Carbon Residue”, Oil & Gas Science and Technology—Rev. IFP, Vol. 52 (1997), No. 1, pp. 73-85.
It would be desirable to provide an improved coking process that enhances the overall efficiency of the preliminary refining process associated with upgrading crude oil and to reduce capital and operating costs for new facilities associated with coking processes of the prior art.
As used herein, the terms “coking unit” and “coker” refer to the same apparatus, and are used interchangeably. The terms “fractionating column” and “fractionator” refer to the same apparatus and are also used interchangeably.